Rename index.css and require.js with md5 hash

- removes obsolete "cache buster" setting
- replaces hardcoded require.js and index.css filenames in settings
files with for non-dev build processes with variables.
- release and couchapp_deploy tasks now rename those two files to
include their hashes, and reference them in the generated index.html
file
You can delete the processed files in the callback of the grunt task, so the grunt task for md5 sums is self-contained. You get the path of the old file passed via `fileChange.oldPath` I think.